FORMATION OF SPINEL IN MELT OF THE MgO—Al2O3—S i O 2—C a F 2 SYSTEM AGGLOMERATED WELDING FLUX AND ITS EFFECT ON VISCOSITY OF SLAG

I.A. GONCHAROV1, V.E. SOKOLSKY2, A.O. DAVIDENKO2, V.I. GALINICH1 and D.D. MISHCHENKO1 1E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine 2Taras Shevchenko National University, Kiev, Ukraine

X-ray examinations of structure of agglomerated flux of the MgO—Al2O3—SiO2—CaF2 system in solid and molten states evidence that in a temperature range above 1200 °C the Al2MgO4 hard spinel phase with a melting temperature of 2105 °C forms in the slag melt. It determines physical-chemical properties of the melt and, in particular, the smooth character of viscosity changes in a temperature range of 1180—1540 °C. By manipulating proportions and concentrations of spinel-forming components Al2O3 and MgO, it is possible to achieve the optimal values of the temperature dependence of viscosity of the slag melt and, on this base, develop welding fluxes with predictable technological properties.

Keywords: welding, agglomerated flux, structure properties in welding of high-strength steels, nor- of slag melts, viscosity, diffraction examinations, spinel mally the use is made of high-basicity fluxes with Conventional-strength steels have been gradu- an increased content of calcium oxide and fluo- ally replaced lately by increased- and high- ride. However, they fail to provide stability of strength steels. To weld such steels it is necessary the welding process and quality formation of the to use a number of appropriate metallurgical welds in multi-arc welding of pipes at a speed of processes and consumables. more than 100 m/h, as they are «short», unlike Welding of conventional-strength steels is the manganese-silicate fluxes. Agglomerated fluxes are finding growing ap- performed by using the MnO—SiO —CaF slag 2 2 plication in welding of high- and increased- system fluxes characterised by good welding- strength steels. This is attributable to their wider technological properties. The latter are achieved metallurgical possibilities of affecting the weld due to the fact that structure of the melts of these pool metal. In general, the problems of sub- fluxes is determined by a silicate skeleton of a merged-arc welding of low-alloy high-strength differing polymerisation degree, which, in turn, steels by using agglomerated fluxes are consid- determines physical-chemical properties of slag ered in detail in studies by I.K. Pokhodnya [2], in the molten state at temperatures of existence D. Olson [3], T. Eagar [4], etc. At the same of the weld pool. To provide formation of the time, there are almost no studies dedicated to defect-free welds in submerged-arc welding the the fundamental research of structure, physical- slag pool should damp oscillations of the metal chemical properties of the slag melts in connec- pool. Therefore, slags must be characterised by tion with technological properties of the welding a smooth increase in viscosity with decrease in fluxes. temperature, i.e. they must be «long» [1]. How- There are several theories describing structure ever, despite good technological properties, the of the welding slag melts. But all of them apply manganese-silicate fluxes have limited possibili- to the equilibrium processes of metallurgy [5—8] ties in terms of affecting the values of mechanical characterised by complete remelting of slag com- properties of the weld metal, and impact tough- ponents and formation of the liquid slag phase. ness in particular. This is explained by the fact Unlike the metallurgical processes, the welding that welding by using these fluxes involves the processes are rapid. They feature substantial tem- processes of formation of silicate non-metallic perature gradients in different zones of the weld inclusions and reduction of silicon in the weld pool and, hence, are far from being equilibrium. metal, which are undesirable for increased- Moreover, in melting during the welding process strength steels. To achieve required mechanical the majority of the agglomerated fluxes used now

© I.A. GONCHAROV, V.E. SOKOLSKY, A.O. DAVIDENKO, V.I. GALINICH and D.D. MISHCHENKO, 2012 18 12/2012 are not a homogeneous liquid, like the fused The E.O. Paton Electric Welding Institute fluxes, but a heterogeneous mixture of different studied the temperature dependence of viscosity phases, some of which are crystalline. At present, of the investigated flux and a number of model there are no slag theories describing the liquid- fluxes of the same slag system in order to deter- crystalline structure of a slag, which does not mine the effect of the solid phase in a liquid slag allow theoretical estimation of the effect of the melt on physical-chemical properties. Viscosity crystalline component of the slag on the processes of slags was measured by using the rotational occurring in the weld pool. viscometer in the Tamman furnace in a purified Coarse crystallites present in the matrix of a argon flow. liquid part of the slag must have a considerable Scanning electron microscope JSM-7700F effect on viscosity and, hence, welding-techno- with the X-ray spectral microanalyser was used logical properties of the flux. Therefore, one of for obtaining images of the initial and remelted the purposes of this study was to qualitatively fluxes, and for local chemical analysis of microin- reveal the effect of the crystalline inclusions on clusions. viscosity of the slag. The powdered specimen of the flux on a graph- For a number of years we have been develop- ite substrate and the solidified specimen of the ing agglomerated welding fluxes based on the slag produced from it by remelting in the molyb- MgO—Al2O3—SiO2—CaF2 slag system. The re- denum crucible at a temperature of 1500 °C in sults obtained prove the possibility of developing the high-purity helium atmosphere were sub- a new generation of welding fluxes based on the jected to electron optical examinations. The slag above system. Study [9] describes investigations specimen was examined on the side of both bot- of structure of a welding flux of the given system tom and surface of the molybdenum crucible. To with a low MgO content (10 wt.%), and study prevent the effect of charging of the specimen by [10] – investigations of the processes of melting the electron beam, a 3 nm thick layer of pure and solidification of fluxes of the above slag sys- platinum was sprayed on the specimen surface. tem. As established in study [11], increase in the Filming of the flux specimen was carried out content of magnesium oxide in fluxes of the at temperatures of 600, 800, 1000, 1200, 1300, MgO—Al2O3—SiO2—CaF2 system leads to de- 1400 and 1500 °C in a high-temperature vacuum crease in thermodynamic activity of SiO2 in the chamber in the high-purity helium atmosphere. slag melt. This fact is very important in terms of Chemical composition of the specimens was metallurgy of welding of high-strength steels, as controlled by the X-ray fluorescent analysis it makes it possible to reasonably suppress the method. undesirable processes of silicon reduction and for- Structural software Powdercell and Mercury, mation of silicate non-metallic inclusions in the data bases Match and Retrieve, which are dis- weld metal. seminated free of charge through the Internet, Experimental conditions and analysis of ex- were used to interpret the X-ray analysis data. perimental data. X-ray examinations of structure The software developed in-house was used in cal- of molten and solid slags were carried out at the culations for investigation of the slag melt [12]. Physical Chemistry Chair of the Taras Shevchen- Investigations at room temperature. The cal- ko National University by using the upgraded culated contents of the main components of the X-ray diffractometer for examinations of melts. flux, i.e. MgO, Al2O3, SiO2 and CaF2, are given Monochromatisation of MoKα-radiation was per- in Table 1. Liquid glass Na—K was added to the formed by using a pair of balanced differential refined mechanical mixture in the granulator. Af- filters Zr—Y. Earlier, the diffactometer was used ter granulation, the flux was held for 24 h at a to advantage for examination of fused fluxes in temperature of 20 °C, and then was baked for glassy and solid states [12]. After upgrading, this 1 h at 500 °C. The actual chemical composition diffractometer allows the diffraction examina- of the flux, according to the X-ray fluorescent tions of specimens to be performed both in the analysis data, is also given in Table 1. liquid and solid states in a temperature range of Results of electron microscopic examinations 100—1700 °C. After granulation and drying of a of the powdered agglomerated flux indicate that flux, it was ground into a powder and subjected Table 1. Composition of welding flux, wt.% to X-ray analysis (CuKα-radiation, diffractome- ter DRON-3M). Also, investigations were car- Type of analysis MgO Al2O3 SiO2 CaF2 Na2OK2OFe2O3 ried out with a solid specimen of the slag after Calculated 30 25 20 25 — — — holding at 1500 °C on the sides of the crucible Actual 25.8 18.7 28.4 22.2 1.58 0.8 1.88 bottom and surface.

12/2012 19 bottom sides differ to some extent. The crystal- line phases formed on the bottom side are finer (Figure 2, spectrum 3). It can be clearly seen that the continuous matrix of one of the phases (the lightest one) contains inclusions of other, finer phases. It is practically impossible to iden- tify this phase as one compound. Most probably, this phase was formed as a result of melting of low-melting point components of the flux and dissolution of some solid components in the melt. If we discard impurities the content of which is less than 2 at.% and ignore a too high concen- tration of fluorine, the darker phase at the centre (Figure 2, spectra 2 and 4) can be identified as Figure 1. Diffraction patterns of agglomerated flux ground Mg2SiO4. The magnified part of image 1 (in black into a powder (points) and summation curve after upgrading frame) is illustrated as electron image 2 in Fi- of profile based on elementary cells of SiO , Al O , MgO 2 2 3 gure 3, where spinel Al MgO in crystallites and CaF2 according to Powdercell (solid line) (1), α-quartz 2 4 (2), corundum (3), periclase (4), and fluorite (5) looking like octahedron, and CaF2 in the form similar to spherulites can be clearly seen (spectra particles of initial components MgO, Al2O3, SiO2 3 and 6). On the bottom side, X-ray spectral and CaF2 hardly undergo any changes as a result analysis fixed no regions with sodium oxide in- of granulation and subsequent baking at a tem- clusions. perature of 500 °C. Examinations conducted on the remelted flux X-ray phase analysis (Figure 1) showed that surface side evidence a more complicated char- after granulation and drying the specimen con- acter of interactions between the components, tained only the initial components, such as α- compared to the bottom side. Inclusions heavily quartz, trigonal Al2O3, and cubic MgO and CaF2. saturated with impurities (Figure 4, spectrum 5) No primary products of solidification of the com- ponents based on the liquid glass were detected. Figures 2—5 show microstructures and data of microanalysis of the bottom and surface of the specimen after remelting in the molybdenum cru- cible. Microstructures on the crucible surface and

Spectrum O F Mg Al Si Ca

1 65.5 1.1 12.8 20.2 0.3 0.2 2 58.3 3.9 14.0 22.2 0.9 0.8 Spectrum O F Mg Al Si Ca 3 44.3 32.8 4.0 2.1 5.4 11.3 1 55.4 13.2 7.5 3.9 10.1 10.0 4 59.6 14.6 7.2 3.0 8.6 7.0 2 54.4 6.2 25.5 0.9 11.6 1.4 5 56.7 12.5 6.8 3.8 10.9 9.3 3 55.3 13.0 7.4 4.3 10.3 9.7 6 44.1 33.3 3.5 2.0 5.0 12.1 4 54.3 6.6 25.0 1.4 11.2 1.5 Figure 3. Microstructure of remelted flux (crucible bottom, Figure 2. Microstructure of remelted flux (crucible bottom, image 2, see Figure 2), and results of X-ray spectral analysis image 1), and results of X-ray spectral analysis (at.%). (at.%). Spectra 1 and 2 can be identified as Al2MgO4, and Spectra 2 and 4 can be identified as Mg2SiO4 spectra 3 and 6 – as CaF2 20 12/2012 can be detected on the surface. It should be noted that sodium is concentrated particularly on this type of the inclusions. Spectrum 2 can be inter- preted as (MgCa)2SiO4, probably with the alu- minium cation impurities, the concentration of fluorine being ignored. Spectrum 3 is closest to CaF2, but with an excess of oxygen. Spectra 1 and 4 in Figure 4 seem to form on a base of the partially solidified low-melting point phase, like spectra 1 and 3 in Figure 2. However, they differ in composition both from each other and from similar images on the bottom side. Of notice is the fact that fluorine in consid- erable amounts is present almost in all reflec- tions, while oxygen exceeds stoichiometric com- positions of the identified phases (see Figures 2— Spectrum O F Na Mg Al Si Ca 5). Spinel crystallites contain the least amounts 1 59.5 10.3 — 11.8 6.2 8.1 4.09 of impurities, although the Al/Mg ratio is about 2 55.1 7.7 — 17.2 4.4 10.6 4.92 1.58 on the bottom side and 1.50 on the surface 3 44.3 32.8 — 4.0 2.1 5.4 11.30 side (in stoichiometric spinel the Al/Mg ratio is equal to 2). Almost all the specimens contain 4 56.5 14.8 — 15.5 2.6 7.2 3.36 no sodium. Most probably, it is concentrated in 5 58.6 17.9 1.3 7.0 2.5 3.8 7.90 sites of accumulation of impurity oxides (Figu- Figure 4. Microstructure of products of solidification of the re 4, spectrum 5), or along the crystalline grain low-melting point phase of remelted flux (crucible surface), boundaries. It should be noted that spectra of and results of X-ray spectral analysis (at.%) the close phases on the bottom side differ from and the normal spinel phase forms. The relative those on the surface side (Figure 5). intensity of this reflection for phases of the spinel The X-ray pattern of the surface side of a type does not exceed 70 % (or much lower) of specimen remelted at a temperature of 1500 °C 100 % reflection (311) in a region of 36.85°. In is shown in Figure 6. The main phases on the our case the most intensive reflection is (440). crucible surface side are cubic spinel (space group Fd—3m (227), a = 0.8100 nm), cubic CaF2 (space group F4/m—32/m (225), a = 0.546 nm) and Mg2SiO4. It is reported that the Mg2SiO4 phase can exist in several modifications. The cubic modification of this phase (of the spinel type) with parameter a = 0.806—0.811 nm is very close to the classic spinel. Diffraction reflections of the spinel form of Mg2SiO4 almost fully coincide with the spinel reflections, providing that they have identical lattice parameters. It is notewor- thy that reflection (440) both for spinel and for Mg2SiO4 is localised in a region of 65.25° to 2θ. However, this reflection is not the most intensive one for these two phases. X-ray phase analysis Spectrum O F Mg Al Si Ca (see Figure 6) confirms the presence of an or- 1 66.8 — 13.2 19.8 0.1 0.1 thorhombic phase (Pbnm) of Mg2SiO4 with a = = 0.4822, b = 1.1108 and c = 0.6382 nm. Also, 2 73.5 — 10.7 15.6 0.1 0.1 there is a phase close to the spinel one for Al2O3. 3 24.1 51.7 1.5 1.5 3.7 17.5 However, the lattice parameter of this spinel is 4 65.6 1.3 21.7 0 11.0 0.3 much lower compared to the magnesium alumi- 5 67.3 — 13.0 19.5 0.1 0.1 nate one (a = 0.7932 nm). It is likely that at the beginning, as the temperature is increased, the 6 67.5 — 12.9 19.4 0.1 0.1 corundum-like lattice of Al2O3 transforms into Figure 5. Microstructure of products of solidification of the the cubic form close to the spinel one, diffusion main part of remelted flux (crucible surface), and results of MgO into such a lattice considerably grows, of X-ray spectral analysis (at.%). Spectra 1, 2, 5 and 6 can be identified as Al2MgO4, and spectrum 4 – as Mg2SiO4 12/2012 21 Figure 2) are characterised by a marked presence of all the elements, and they are hard interprete unambiguously. It is likely that this is the phase that formed from the melt and had no time to solidify by preserving a partially amorphous state. No phases with a close composition were fixed in the diffraction patterns. Therefore, the spinel octahedrons (see Figure 3, spectra 1 and 2), Mg2SiO4 (see Figure 2, spectra 2 and 4), CaF2 in the formations looking like spherulites and the liquid-type phase were unambiguously interpreted on the bottom side. The X-ray pattern of the remelted flux on the bottom side is hard to interpret. Almost every Figure 6. Diffraction patterns of the flux remelted at 1500 °C on the surface side: 1 – experiment and summation peak has strong asymmetry on the side of small curve of spinel Al2MgO4, calcium fluoride and Mg2SiO4; scattering angles, this evidencing a poor phase 2—4 – upgraded profiles, according to PCW, of spinel formation on the bottom side. For example, the Al2MgO4 (2), CaF2 (3) and Mg2SiO4 (4) phases close in structure, i.e. the spinel form of It seems that this possibility exists when at first aluminium dioxide, Mg2SiO4, and high- and low- one of the phases forms, and when the other phase temperature spinels, which have the same struc- starts growing epitaxially on one of its faces, or, tural type of lattice parameters a, i.e. Fd—3m, which is more probable, when in formation of but can range from 7.90 to 8.20 nm, persist on the liquid phase together with the crystalline the bottom side. In general, the phases appearing spinel its crystallites become oriented in a certain on the bottom side are poorly formed and hard direction relative to the melt surface. It should to distinguish. X-ray analysis confirms the pres- ence of spinel Al MgO , Mg SiO and CaF . be added that the Al2MgO4 spinel crystallites in 2 4 2 4 2 the form of black octahedrons are unambiguously High-temperature X-ray solid-state exami- interpreted by electron microscopy analysis (see nations. To trace the sequence of solid-state re- Figures 3 and 5), and the phase close to spinel actions, the ceramic flux ground into a powder was placed into a molybdenum crucible, which Mg2SiO4 is not solidified in the form of octahe- drons. X-ray spectral analysis confirms the pres- was mounted on a work table inside a high-tem- perature vacuum chamber of the diffractometer ence of Mg2SiO4, probably in the (CaMg)2SiO4 form (see Figures 2 and 4). for examinations of melts [12], and subjected to Spinel is not the main phase on the bottom high-temperature X-ray phase analysis. As shown side. Moreover, it was difficult to detect spinel by the analysis, structural changes in a specimen on the bottom side because of its low concentra- occur over the entire temperature range (Figu- tion. Figure 2 shows probably the only spinel re 7). Up to 1200 °C, they take place in the solid phase found at the bottom, and in Figure 3 it is phase. At low temperatures, the structural shown on an increased scale. Spectra 1 and 3 (see changes are slow and occur mainly inside the phases. For example, the most intensive peak corresponding to the 100 % intensity of hexagonal α-quartz dramatically decreases already at 600 °C. At the same time, there appear the peaks corresponding to other modifications of silica. Solid-state interactions between the phases begin at higher temperatures. It should be taken into account that the main components in the agglomerated flux are contained in the matrix of a binding material (Na—K liquid glass annealing product). Formation of vacancies in the matrix during evaporation of water will lead to the fact that the inter-phase interaction will take place

Figure 7. Diffraction patterns of the flux (MoKα-radiation) due to diffusion of atoms of the flux components at room temperature (1), 600 (2), 800 (3), 1000 (4), 1200 into the matrix at a rather high rate. Probably, (5), 1300 (6), 1400 (7) and 1500 (8) °C. Diffraction pattern nuclei of the new phases, which can partially of pure spinel (points) is superimposed on diffraction pat- dissolve at the initial stage of melting of some tern at 1300 °C. Diffusion background is fully removed 22 12/2012 flux components in the molten phase, or, on the in the entire concentration range of the consti- contrary, can accumulate due to intensified in- tutional diagram at a temperature of up to teraction with the molten phase, will start form- 2000 °C. Investigations of the solid solutions of ing in the matrix on a base of the liquid glass Al2O3 in the spinel show that they have cation remainders. Below 1200 °C, the melts preserve vacancies, which are readily filled up from ex- the main components, although, as can be seen ternal sources. Reaction of formation of the spinel from Figure 7, intensive decrease in their peaks in the solid phase from aluminium oxide begins and, hence, destruction of the initial structures approximately at 750 °C [15]. According to [13], take place. At 1200 °C, the diffraction patterns the initial stage of formation of the spinel occurs show practically no reflections corresponding to by the surface diffusion of alumina into MgO, the initial flux components. The flux melts. How- which is followed by the volume diffusion of ever, a new crystalline phase, i.e. spinel atoms (ions) of magnesium and aluminium Al2MgO4, forms against the background of the through the oxygen sub-lattice to transform the molten component. hexagonal arrangement of this sub-lattice in co- The diffraction pattern of pure spinel is su- rundum into the cubic one (probably, the corun- perimposed on the diffraction pattern of a speci- dum itself first forms the cubic lattice of the men at a temperature of 1300 °C (see Figure 7). spinel type). A high melting temperature of Then the spinel reflections in a temperature range spinel and physical-chemical properties of the of 1200—1500 °C coincide in location with the slag melts it determines arouse considerable in- specimen reflections. The relative intensity of the terest in the spinel when developing a new gen- reflections is in some disagreement with the tabu- eration of welding fluxes. The spinel phase per- lar data. This can be the case if the crystallites sists after remelting of the flux during the weld- of spinel in the melt are oriented with a certain ing process and its transformation into a slag plane relative to the melt surface, rather than crust. being chaotically located. As the melt surface is Therefore, the presence of spinel is an experi- saturated with spinel to a much greater degree, mental fact, and it is necessary to reveal all ad- and the bottom – to a lesser degree, it can be vantages and drawbacks of the presence of a solid assumed that the spinel is lighter than the liquid spinel phase in the melt, as well as its effect on phase and, as such, it floats to the surface. the welding process. It is possible to distinguish Discussion of results. The insignificant ad- four moments which can be affected by formation ditions the content of which is not in excess of of the crystalline spinel, although, in general, 1 wt.% being ignored, to interpret the results there may be much more of such moments. obtained it is necessary to use a complex consti- 1. If the thermal expansion coefficients of tutional diagram of Na2O—Al2O3—MgO—SiO2— spinel and liquid phase solidification products CaF2. But this oxide-fluoride diagram is not differ greatly at the temperatures of detachment available in the scientific literature. The closest, of the slag crust from the weld metal (250— well-studied constitutional diagram for the four- 500 °C), this will induce microstresses in the ma- component system is CaO—MgO—Al2O3—SiO2 trix of the former liquid phase, thus creating [13]. As discovered in investigation of the ag- preconditions for good detachability of the slag glomerated flux of the calculated MgO crust from the surface of the metal welded. (10 wt.%)—Al2O3 (25 wt.%)—SiO2 (40 wt.%)— 2. The presence of the crystalline phase in the CaF2 (25 wt.%) composition, CaF2 at least par- liquid slag melt at a temperature of about 1100— tially transforms into an oxide [9, 14]. It was 2000 °C will have a considerable effect on vis- established for this flux that the main phase has cosity of the melt. Increase in the content of the the form of anorthite, i.e. CaAl2Si2O8, where crystalline spinel in the molten slag may lead to calcium has the oxide form. Probably, MgF2 increase in viscosity and make the molten slag forms in small amounts. Spinel Al2MgO4 does «longer», this being a positive factor in terms of not form in this case. The spinel fields appear in the technological properties of the flux, which the CaO—MgO—Al2O3—SiO2 diagram already in was already noted at the beginning of the article. a section of 15 wt.% Al2O3. In the MgO—Al2O3 3. The general decrease in the content of elec- binary constitutional diagram the spinel is almost tricity carriers in the slag during formation of stoichiometric at low temperatures, and features the spinel phase due to decrease in the quantity a wide range of homogeneity at high tempera- and mobility of elementary particles (ions and tures. Earlier the spinel was not distinguished electrons) may lead to a substantial decrease in from the cubic modification of Al2O3. Solid so- electrical conductivity. This also is a positive lutions based on spinel MgO and Al2O3 are found factor, as it will allow mitigating the effect of

12/2012 23 Table 2. Calculated compositions of model welding fluxes, wt.%

Flux number MgO Al2O3 SiO2 CaF2 MgO + Al2O3 34030151570 43025202555 6 40 0 35 25 — 12 40 10 40 10 50 15 40 25 10 25 65 20 35 0 40 25 —

are characterised by a jump-like change in vis- cosity in a temperature range of 1150—1250 °C. These are the so-called «short» fluxes. Fluxes 3 and 15 with 40 wt.% of magnesium oxide and 25—30 wt.% of aluminium oxide are characterised by a high viscosity over the entire range of the studied temperatures. The absence of the flat part on the viscosity curve is indicative of an incom- Figure 8. Temperature dependence of viscosity of model plete melting of these slags and presence of a slags of the MgO—Al2O3—SiO2—CaF2 system (numbers of large number of solid particles in the melt, which curves correspond to numbers of fluxes in Table 2) could be observed in the course of the experiment. shunting of the current through the slag and im- Most probably, this phase is Al2MgO4. Flux 4 proving stability of the process in case of multi- investigated here has a somewhat lower calcu- arc welding. lated content of magnesium oxide (30 wt.%), 4. The presence of the solid phase in the slag and the content of aluminium oxide in it is melt will reduce thermodynamic activity of some 25 wt.% (see Tables 1 and 2). This flux is char- components of the flux and, hence, affect its met- acterised by a rather smooth change in viscosity allurgical properties, as was confirmed by the over the entire temperature range (see Figure 8; investigations [11]. flux 4 in Table 2). In this case the curve comprises The most important factor in terms of ensuring some «steps», which can be explained by solidi- the required technological properties of the weld- fication of extra portions of the Al2MgO4 solid ing flux is a temperature dependence of its vis- phase from the melt. As to the character of the cosity. Several model agglomerated welding temperature dependence of viscosity, flux 4 is fluxes of the investigated system were made for close to manganese-silicate flux AN-60. Welding- further investigations of viscosity. The calculated technological tests of the fluxes indicated in Ta- compositions of the fluxes are given in Table 2, ble 2 showed that flux 4 has the best properties. and the viscosity measurement results are shown Therefore, it is the presence of the solid phase in in Figure 8. It was found that the intensity of the slag melt, in our opinion, that determines reflection of the crystalline spinel decreases from the character of the temperature dependence of specimen to specimen in the following order: 3 → viscosity. It is likely that by manipulating with → 15 → 4 → 12, this being in correlation with the proportions and concentrations of spinel com- decrease in sum of the spinel components, i.e. ponents Al2O4 and MgO it is possible to achieve the required viscosity values and character of the MgO + Al2O3 (see the last column in Table 2). In our opinion, the sum of the spinel components temperature dependence, and, on this base, de- in the first approximation can serve for quanti- velop fluxes with the controllable viscosity and tative evaluation of the spinel until the other predictable technological properties. We believe criterion is found, which can correlate with the that fluxes, in the melts of which the crystalline content of the crystalline spinel in a specimen. spinel forms, will combine good technological In specimens 6 and 20, no spinel was detected and metallurgical properties. because of the absence of Al2O3, and its content in specimen 12 is too low because of a low content CONCLUSIONS of Al2O3. 1. X-ray examinations of structure of the MgO— It was established that fluxes (6, 12 and 20) Al2O3—SiO2—CaF2 system agglomerated flux in with the content of magnesium and silicon equal the initial, molten and solid states, after holding to 35—40 % and aluminium oxide equal to 10 % at 1500 °C, were carried out. Images of the initial 24 12/2012 and remelted flux were obtained by using scan- aminations and assistance in interpretation of ning electron microscope JSM-7700F fitted with the obtained results. the X-ray spectral microanalyser, and local 1. Podgaetsky, V.V., Lyuborets, I.I. (1984) Welding chemical compositions of the microinclusions fluxes. Kiev: Tekhnika. were determined. Viscosity of the slags was meas- 2. Pokhodnya, I.K., Yavdoshchin, I.R., Shvachko, V.I. ured by using the rotational viscometer in a pu- et al. (2004) Metallurgy of arc welding. Interaction of gases with metals. Ed. by I.K. Pokhodnya. Kiev: rified argon flow inside the Tamman furnace. Naukova Dumka. 2. The examinations conducted evidence a 3. Olson, D.L. (1983) The investigation of the influ- ence of welding flux on the pyrometallurgical, physi- complex character of interactions taking place in cal and mechanical behavior of weld metal. In: Final the agglomerated flux before formation of the report of Center for welding research Colorado molten slag phase. The main structural changes school of mines Golden Colorado. 4. Eagar, T.W. (1991) Thermochemistry of joining. In: in heating the flux to 1200 °C occur due to the Proc. of Elliot Symp. on Chemical Process Metal- solid-phase interactions in a product formed by lurgy. a cake of the liquid glass with the adjacent main 5. Herasymenko, P., Speigth, G.E. (1950) Ionic theory of slag-metal equilibrium. J. Iron and Steel Inst., components of the flux. 166, 169—183, 289—303. 3. Formation of the liquid phase begins in a 6. Tyomkin, M.I. (1946) Mixtures of molten salts as ionic solutions. Zhurnal Fizich. Khimii, Issue 1, temperature range of about 1200 °C due to melt- 105—110. ing of the cake of the liquid glass with the main 7. Kozheurov, V.A. (1955) Thermodynamics of metal- components and complex unstable compounds. lurgical slags. Sverdlovsk: Metallurgizdat. 8. Esin, O.A. (1961) About the nature of molten sili- The solid Al2MgO4 spinel phase having a melting cates. Tr. Ural. Polytechn. In-ta, Issue 122, 29—39. temperature of 2105 °C forms in this case in the 9. Sokolskii, V.E., Roik, A.S., Davidenko, A.O. et al. (2012) X-ray diffraction and sem/edx studies on melt. Full melting of the flux of the investigated technological evolution of the oxide-fluoride ceramic composition does not take place in a temperature flux for submerged arc-surfacing. J. Min. Metall. B, range of up to 1500 °C. This phase persists in the 48, 101—113. 10. Duchenko, A.N., Mishchenko, D.D., Goncharov, I.A. slag crust after remelting of the flux during the et al. (2011) Study of the process of melting and soli- welding process. The presence of the solid dification of fluxes of the Al2O3—MgO—SiO2—CaF2 slag system. In: Proc. of 13th Rus. Conf. on Structu- Al2MgO4 phase in the slag melt of the MgO— re and Properties of Metallic and Slag Melts. Al2O3—SiO2—CaF2 system determines its physi- Vol. 3: Experimental investigation of slag melts, cal-chemical properties, in particular the smooth metal-slag interaction (Ekaterinburg, 2011), 136—139. character of viscosity changes in a temperature 11. Goncharov, I.A., Galinich, V.I., Mishchenko, D.D. range of 1180—1540 °C. et al. (2011) Prediction of thermodynamic properties of melts of MgO—Al2O3—SiO2—CaF2 system. The 4. It is an established fact that by manipulat- Paton Welding J., 10, 2—4. ing the proportions and concentrations of spinel- 12. Shpak, A.P., Sokolsky, V.E., Kazimirov, V.P. et al. forming components Al O and MgO it is possi- (2003) Structural peculiarities of oxide system melts. 2 3 Kiev: Akademperiodika. ble to achieve the optimal values of the tempera- 13. Berezhnoj, A.S. (1970) Multi-component oxide sys- ture dependence of viscosity of the slag melt and, tems. Kiev: Naukova Dumka. on this base, develop welding fluxes with con- 14. Sokolsky, V.E., Roik, A.S., Davidenko, A.O. et al. (2010) On phase transformations in agglomerated trollable technological properties. flux of salt-oxide slag system at heating. The Paton Welding J., 12, 9—14. The authors express their gratitude to Com- 15. Pashchenko, A.A., Myasnikov, A.A., Myasnikova, E.A. et al. (1986) Physical chemistry of silicates: pany «Tokyo Boeki» and personally to Dr. Tin- Manual. Ed. by A.A. Pashchenko. Moscow: Vysshaya kov V.A. for conducting electron microscopy ex- Shkola.

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